With the passing of Tony Britton in December 2019, actor Melvyn Hayes, who played Willem, is now the sole surviving cast member.
Exteriors filmed on location in Amsterdam and on the Dutch coast. This is unusual for an entirely British made and financed film of this period as most UK productions tended to use studio sets and UK locations to double for foreign places due to budgetary constraints.
The film opens with the Nazi invasion of Holland on Friday 10th May 1940, Winston Churchill's rise to British Prime Minister the following day, and then the launch of Operation Amsterdam itself on Sunday 12th May.
Melvyn Hayes played a young boy but was 24/25 at the time. Melvyn continued to play roles younger than his real age for much of his early to mid career, in part due to his height of 5'3" (1.6m). He is 88 and pops up now and again in interviews.
As the Colonel (John Le Mesurier) comes to their rescue from the fifth columnists, he waves them on but is not seen from the front. He salutes as the car passes but the front view of the actor is filmed in front of a projection. All the other scenes he was in were clearly shot on sets.